The present invention relates to nuclear magnetic resonance (NMR), particularly to the use of NMR for the analysis of thin planar or curved layer and surface layer geometries, and more particularly to a sensor using a miniaturized meanderline surface coil for NMR analysis of contact layers, non-destructive depth profiling to films, or imaging thin multilayers in a 3-D sense. The invention enables high resolution NMR of the chemistry and physics of surface layers, or layered materials, without the interference of signals from the bulk material.
The nuclear magnetic resonance (NMR) and the magnetic resonance imaging (MRI) technologies are well known. NMR and MRI are normally considered to be bulk techniques for examining short range structure and dynamics in liquid and solid materials. Surface NMR has been observed for absorbed species and surfaces of high surface area powders. Optically detected NMR and surface NMR with polarized atomic beams have sufficient sensitivity to detect signals from flat surfaces but are of low resolution, limited to relaxation studies, and not found widespread use due to their complexities. For the most part, magnetic resonance experiments are usually carried out in the presence of a spatially uniform static DC (direct current) magnetic fields and a spatially uniform radio frequency (RF) excitation fields. In the case of NMR, particle diffusion measurements are generally carried out by applying an inhomogeneous static field rather than using an inhomogeneous RF field. An RF excitation, with a wave vector periodically varying across the plane of the surface of a sample, may be generated by a periodic meanderline.
A meanderline is a zig-zag or serpentine array of parallel conductors of mutual separation, and has been used for several years as an electromagnetic acoustic transducer and has RF magnetic field characteristics which are well understood. Meanderlines have been used for excitation of bulk and surface spin waves in ferromagnets, see S. A. Bogacz, et al. "New Techniques For Excitation of Bulk and Surface Spin Waves in Ferromagnets," J. App. Phys. 58(5), Sep. 1, 1985, and to search for nuclear acoustic resonance effects on surface waves in films, see R. G. Spulak, Jr., "Detection of Alpher-Rubin attenuation and a search for nuclear acoustic resonance of surface waves in tantalum films," Phys. Rev. B, 40(9), Sep. 15, 1989. Also, a meanderline surface coil has been utilized in detecting .sup.14 N pure nuclear quadrupole resonance (NQR) signals in large thin-layer samples or in regions near the surface of bulk samples while avoiding interfering signals from interior regions. See M. L. Buess, et al., "NQR Detection Using a Meanderline Surface Coil," J. Magn. Resonance 92, 348 (1991), and the meanderline coil thereof, which demonstrated its use for observation of zero-field pure NQR in large samples such as 100 cc of Na.sup.14 NO.sub.2 at 3.6 MHz, and the meanderline thereof is illustrated and will be discussed hereinafter with respect to FIGS. 1 and 2.
In the present invention, the meanderline surface coil, such as shown in FIGS. 1 and 2, has been miniaturized to small planar (or cylindrically curved) samples and applied to the much more useful NMR technique by determining the unique configurations in an external magnetic field which produce the uniform orthogonal RF pulses and allow the magic angle spinning required for high-resolution spectroscopy. Thus, the miniaturized sensor coil of this invention extends the capabilities of NMR to provide analysis of thin planar (or curved) samples and surface layer geometries.